1. Field of the Invention
The present invention relates to a choke coil, and more particularly to a common mode choke coil for removing common mode noise.
2. Description of the Related Art
An alternating-current power supply line constitutes a route through which external noises flow into an electronic apparatus or noises generated inside an electronic apparatus flow out. To cope with this, a choke coil is inserted in series in an alternating-current power supply line for cutting off the noises. There are two kinds of noises; a normal-mode noise generated between lines and a common mode noise generated in both lines relative to the ground, and in general the common mode noise causes problems. A common mode choke coil for removing the common mode noise is required to have a large inductance for the common mode noise and to cause magnetic fluxes to be cancelled out with each other for the alternating voltage of a commercial power supply.
In a case where a toroidal magnetic core is used for a magnetic core of a common mode choke coil, the amount of leakage flux is small with a bifilar winding in which two wires are wound together. However, when a high voltage is applied between two coils, the two coils have to be apart from each other in view of insulation between the coils, and therefore a bifilar winding can not be applied. Thus, it is a common practice that the two coils are wound separately, which causes the following problems.
FIGS. 7A and 7B are a front view and a side view of a conventional toroidal common mode choke coil of vertical type, respectively. FIGS. 8A and 8B are a plan view and a side view of a conventional toroidal common mode choke coil of horizontal type, respectively. In FIGS. 7A, 7B, 8A and 8B, a plastic insulating partition plate 2 is provided within a toroidal magnetic core 1 and attached to an inner circumference of a plastic insulating case 5 housing the toroidal magnetic core 1, and two coils 3a, 3b wound in the same direction are provided so as to sandwich the plastic insulating partition plate 2. In FIGS. 7A and 7B, the magnetic core 1 is vertically mounted on a plastic base 4, while in FIGS. 8A and 8B, the magnetic core 1 is horizontally mounted on a plastic base 4, and in both cases the respective windings are constructed to lead out from terminals provided on the plastic base 4.
FIGS. 9A and 9B are a plan view and a side view, respectively, showing leakage flux from the toroidal common mode choke coil. As shown in FIGS. 9A and 9B, when an alternating-current power supply current i flows, magnetic poles each having a same pole and a same magnetic potential are generated at a portion A and a portion B of the toroidal magnetic core, respectively, which are ends of the two coils (in the figure, the portion A is shown as an N pole, while the portion B as an S pole).
Due to the generation of the magnetic poles, a part of the magnetic flux turns into leakage flux "PHgr"R travelling from the portion A (N pole) through a space in the vicinity of the toroidal magnetic core to the opposite portion B (S pole). The direction of the generated magnetic flux changes to the direction of current which alternates in accordance with the frequency of a connected power supply. The leakage flux travelling in the space adversely affects peripheral electronic components, in particular, a CRT display or the like thereby disturbing pictures, and lots of efforts have been made for countermeasures.
In order to solve the problem, the mounting position on a substrate of the toroidal common mode choke coil from which the leakage flux is originated is discussed, the coil is entirely covered with a magnetic shield cover, or a magnetic shield plate is provided to cut off the leakage flux when incorporated in an electronic apparatus.
As shown in FIG. 10, a common mode choke coil or the like which generates leakage flux is entirely covered with a magnetic shield cover 7, and the cover is fixed to a base 4. Alternatively, as shown in FIG. 11, a toroidal common mode choke coil 9 generating leakage flux is covered with a magnetic shield plate 10 when mounted on a substrate 8.
FIG. 12 shows an example of a conventional EE-type common mode choke coil. A magnetic core 11 consists of two E-shaped ferrite magnetic cores, and two coils 12a, 12b are wound around a central bar thereof in phase. Magnetic poles of the two coils generated by an alternating-current power supply current agree with each other at a portion C or portions D, E of the EE-type magnetic core each other, and magnetic fluxes shown by dotted lines designated by "PHgr"1 and "PHgr"2 flow within the magnetic core. Thus, according to the construction of the EE-type common mode choke coil, the amount of magnetic flux leaking outside is smaller compared with the toroidal magnetic core. Therefore, the EE-type magnetic core common mode choke coil is more advantageous in leakage flux than the toroidal common mode choke coil.
As has been described above, the toroidal common mode choke coil is small in size but has a large amount of leakage flux generated, which adversely affects an external apparatus, and as a countermeasure against this, a shield cover, a magnetic shield plate or the like is needed. This pushes up the cost due to materials needed, and therefore an inexpensive and practical countermeasure against leakage flux has been demanded.
Further, although the EE-type magnetic core common mode choke coil is more advantageous in leakage flux than the toroidal magnetic core common mode choke coil, the EE-type magnetic core has a longer magnetic path compared with the toroidal magnetic core, so the number of windings has to be increased in order to gain the same inductance. Due to this, a larger magnetic core and more copper as wire material need to be used. This consumes lots of resources giving a heavier load to the environment for obtaining the same function, and at the same time increases the cost for raw materials. Additionally, the increase in the number of windings increases the winding resistance, resulting in a larger power consumption when the common mode choke is driven. In addition, in the case of the EE-type magnetic core, since a winding is provided on a bobbin in layers, heat generated is confined therein. Moreover, when a common mode choke coil is required to have a large rated current and a large inductance as well, a plurality of such EE-type common mode choke coils need to be connected in series so as to increase the inductance.
An object of the present invention is to realize a common mode choke coil with solutions to the above problems, which can provide low-cost and practical countermeasures against leakage flux and can eliminate any adverse effect on an external apparatus.
With a view to attaining the above object, according to a first aspect of the present invention, in a common mode choke coil in which one coil is wound around one half of a toroidal magnetic core mounted on a base and another coil is wound around the other half thereof such that the both coils are wound equally so as to cancel out respective magnetic fluxes relative to a normal mode current, a belt-like magnetic shield plate is provided along a direction of leakage flux generated from the two coils so as to reduce the amount of the leakage flux.
According to a second aspect of the present invention, in the common mode choke coil, a partition plate is provided inside the toroidal magnetic core for isolating the two coils from each other, and the belt-like magnetic shield plate is substantially U-shaped and placed in a direction along the partition plate so as to cover the toroidal magnetic core.
According to a third aspect of the present invention, in the common mode choke coil, the magnetic shield plate is attached to an insulating protection plate on a face opposite to a face facing the coils.
According to a fourth aspect of the present invention, in the common mode choke coil, the insulating protection plate to which the magnetic shield plate is attached is detachably attached at both ends thereof to the base on which the toroidal magnetic core is mounted.
According to a fifth aspect of the present invention, in the common mode choke coil, the insulating protection plate to which the magnetic shield plate is attached has side walls provided along the magnetic shield plate.
Since the present invention is constructed as described above, the magnetic shield plate functions to reduce the amount of magnetic flux leaking from the toroidal magnetic core.
Additionally, the insulating protection plate provided between the magnetic shield plate and the coils functions not only to reinforce the magnetic shield plate but also to increase the dielectric strength between the coils wound around the toroidal magnetic core and the magnetic shield plate by providing the side walls which increase the creeping distance.
Then, the ends of the substantially U-shaped insulating protection plate function to allow the plate to be detachably attached to the base.